1. Field of the Invention
The invention relates to a method for converting printing data and a device using the same, which apply to output devices for digital images such as a printer, and in particular to a method for converting printing data and an applied device which directly generates printing data of any resolution and output it.
2. Related Art
With the development of the digital technology, the applications for the digital images are getting broader. Digital images are easy to achieve by digital cameras and scanners. The images are output by an image output equipment such as a digital photo printer in which a memory card slot and a LCD display are included inside. Generally, most kinds of the memory cards shown in the market can be supported so the computer is not the only interface for printing any more. The only thing you need to do to perform print job is plugging the memory card into the slot on the printer and pressing the buttons thereon. The whole procedure becomes much more convenient and faster.
However, digital images usually are composed of trichromatic colors: red (R), green (G), and blue (B) in different proportions. However, a lot of image output devices and displays are not able to produce the enough tints. A digital image must be undergone a color conversion to transfer to a color space for output device before printing. Taking a printer as an example, the color space is formed by cyan (C), magenta (M), and yellow (Y). After that, the image is undergone a down tint process for converting the original multi tints image into a less tints image in order to conform to the characteristics of output devices. This step can be referred to as a halftoning process. The values of the pixels in the halftoning images are responded to drop numbers of the ink. Finally, a halftoning image is converted and output according to the arrangement of the ink nozzles of the printhead for producing a printed image.
Associated with the advancement of the printing systems and printheads, the resolution for printing has been improved from 300 DPI ten years ago to 4800 DPI presently. Generally, the resolution of a digital image is determined at the time of shooting or scanning. The only thing that can be changed during the printing process is the printing resolution. However, the size of a printout is changed with the variation of printing resolution, which means a higher printing resolution gets a smaller image and vice versa. Thus, an additional process is required for outputting an image with a fixed size when the printing resolution is changed. On the other hand, color-related experiments are needed for a printmode with varied resolutions to obtain the relationship between the halftone levels and drop numbers of the ink, which cost a lot of labor and time, especially for a printmode with a higher resolution.
In order to solve the foregoing problems, in the technique disclosed in the U.S. Pat. No. 5,480,240, the original 300 DPI printing data are directly repeated in the horizontal direction to make one dot become two dots. To retain the characteristics of original printing data, a correction process will be performed to the doubled printing data. This correction process is a dot removing process, which prevents two adjacent dots from printing at the same time, and forms the virtual 600 DPI printing data. This method can only achieve the 600 DPI printing in the horizontal direction, and the resolution of the data is not a real 600 DPI according to a strict view.
U.S. Pat. No. 5,742,300 is also for enhancing the printing resolution in the horizontal direction, where the data are firstly doubled in the horizontal direction to make one dot become two dots. And then, a dot removing process is performed to the edge for the blur effect caused by the doubled printing data. For the regions beside the edges, a checkerboard-like mask is used to reduce the amounts of the printing data, which makes the printing ink as similar as possible before and after the resolution enhancement. This method only improves the resolution in the horizontal direction, and an additional image edge detection process makes this process even more complicated.
In addition, U.S. Pat. No. 5,912,683 also provides the method for improving a printing resolution in the horizontal direction. Instead of directly doubling the printing data, a transition table is utilized to replace a dither table in the halftone process. The same pixel in the original image is continuously compared to the two thresholds in the transition table, twice in horizontal direction. Therefore, two halftone outputs are obtained by the same input pixel. This method is limited by using the dithering method to perform the halftone processing, which achieves halftoning images with poor quality and may even cause ink bleeding and color shift due to the increase of the ink drops per area.
In U.S. Pat. No. 6,075,926, duplication or average method is used to increase the dots in the low brightness area, which is less sensitive to human eyes. In the area where the brightness is higher than a threshold value, a table consulting method is used to insert some dots. Firstly, a database including different features of images is produced by experiments where four filter matrixes required by an operation of generating four dots from one dot are represented. In a practical operation, the processing dot is treated as the center of the 9×9 matrix; an eight dimension vector is obtained by subtracting the processing dot from the surrounding eight dots respectively, which is used for searching for one or several sets of filter matrixes in the database by matching operation, which corresponds to the features similar to the processing dot. Finally, the value of the inserting dot is calculated by adding the multiplications of the weighted ratios and the filter matrixes. In this method, the database must be established and stored in the memory when the system is offline. Although a larger database achieves a better image quality, the larger storage size and the longer matching time are needed. On the other hand, different databases are needed for different multiple of resolution conversion, which means a large memory is necessary.
In U.S. Pat. No. 6,411,399, the problem caused by doubling the original printing data according to the desirable printing resolution is mentioned, which is that the amount of ink printed on the medium become double such that the color presentation is even worse. Therefore, the mask changed with the printing position is used to remove the redundant printing data to make the amount of ink remain the same with the variation of resolution. Although this method makes the amount of the ink remain the same, the printing data will be changed with the position. In other words, an ink drop originally located in the center will be dispersed to the surrounding location, which makes the printing quality become worse. In this method, color bleeding still happens as two adjacent dots are printing at a high resolution.
According to the description above, when the image is printed at printmodes with different resolutions, the pre-requited experiments for the relations between the halftone levels and drop numbers of ink must be performed. On the other hand, if images stored in a memory card are printed, the size of the printouts will be changed with the printing resolutions. Thus, a new method for converting the printing data and an applied device are necessary to develop to print images at different resolutions and to save the labors and time required in the color experiments.